1. Field of the Invention
The present invention relates to a magnetic head apparatus and, more particularly, to a magnetic head apparatus mounted on, for example, a video cassette recorder (VCR).
2. Description of the Related Art
When a VCR is operated in a reproducing mode, so called noise bars may be generated in a still image or an image at the time of a special reproducing mode, such as the one third speed reproduction mode. To prevent noise bars, the VCR has a magnetic head apparatus in which its head is driven in a direction substantially perpendicular to the head trace direction. In such an apparatus, the magnetic head is mounted on the distal end of a piezoelectric element comprising a pair of piezoelectric members, which are ceramic and attached to each other by an adhesive. The piezoelectric element is known as a bimorph element, in which one of the piezoelectric members contracts and the other expands when a driving voltage is applied. As the piezoelectric members contract and expand, respectively, the magnetic head is driven in the direction substantially perpendicular to the head trace direction.
In general, the ceramic which is used to form a piezoelectric element is difficult to treat and easy to break during the manufacturing process. Hence, an element of a desired shape cannot be obtained, and therefore the magnetic head cannot be driven or controlled accurately.
To reinforce the element, a thin metal plate can be interposed between the piezoelectric members. Recently, a piezoelectric element has been developed which comprises a pair of piezoelectric members and a thin metal plate interposed therebetween. The thin metal plate is made of 42Ni-Fe having a coefficient of thermal expansion of 40.times.10.sup.-7 (.degree.C).sup.-1, which is substantially the same as the coefficient of thermal expansion of the piezoelectric element, i.e., 20 to 40.times.10.sup.-7 (.degree.C).sup.-1. 42Ni-Fe is made from Invar, which is known as a material having a small coefficient of thermal expansion, by slightly changing its composition. Such a magnetic head apparatus is disclosed, for example, in U.S. Pat. No. 4,151,569, in which a brass plate is interposed between two piezoelectric ceramic members. By use of such a thin metal plate, the piezoelectric members are reinforced, resulting in a stronger piezoelectric element as a whole. In addition, since the thin metal plate can be subjected to etching and press working, the manufacturing accuracy of head apparatuses has been considerably increased.
However, since 42Ni-Fe is a magnetic substance, the metal plate made of this material may be magnetized by the influence of an external magnetic field. For this reason, the magnetic head provided on the piezoelectric element having a 42Ni-Fe plate also tends to be magnetized. This tendency degrades the characteristic of the magnetic head, and the magnetic head apparatus is of no practical use.